Paper summary – de Queiroz 1998

I managed to volunteer myself to lead the first discussion for Season 2, so thought I would write down a brief summary of the paper and a few of my thoughts on it as a spur to conversation. If you haven’t caught up with it, here’s the details of the paper we decided on for the first session

There are many species concepts, but one definition?

Everyone who has done an undergraduate course in biology has had the “species concept” discussion. Although species are fundamental units in biodiversity, and often the natural unit of comparison in evolutinary and ecological studies, bioloigsts spend quite a lot of their time arguing about exactly what constitutes a species. John Wilkins, who knows a thing or two about the debate, lists 26 species concepts that have at least one supporter. de Queiroz provides a summary of the more popular concepts, and the evidence they use to delimit species.

The huge number of species concepts available seems at first to be a barrier to the important tasks of delimiting species (alpha taxonomy) and working our how they came to be (the study of speciation). But, de Quieroz argues, all these concepts can be shown to share a single definition of species, and only differ in determining the types of evidence we should look to in deciding if a population fits that definition:

All modern species definitions either explicitly or implicitly equate species with segments of population-level evolutionary lineages.

For de Quieroz a species is a population, held together by some force, evolving through time, and all the existing species concepts already recognise this idea. In sexually reproducing organisms, it is reproduction itself that can pull partially-isolated populations in a single direction. de Quieroz doesn’t suggest similar forces that might apply in asexual species, but notes existing species concepts for these species presume such forces exist.

Linking species status to the speciation process

For me, the strongest and most interesting part of the general lineage species concept (GLSC) is the way it brings what we know about speciation, the study of the origin of species, into taxonomy, the process by which we identify and describe species. de Quieroz illustrates a hypothetical speciation proceeds something like this (this is actually my version, lifted from a powerpoint slide, so blame me if it’s misleading):

The point here is not the speciation will allow follow this path, but that speciation is fundamentally a process not an event, and the criteria required to fulfill various species concepts will accrue during that process. The choice of a species concept may depend on exactly what is being studied, and a more general approach to taxonomy should take all the evidence available.

Some thoughts

This paper is now 15 years old, and the ideas it contains have had a large impact on both speciation research and taxonomy (it’s been cited almost 600 times). I’m sure we’ll have no shortage of things to talk about, but here’s a couple of things that nag away at me when I think the GLSC is the answer to all our problems

Applicability outside of living metazoans

I only work on animals that are alive today. Can the GLSC be applied to asexual populations, or species that arise from hybridisation rather than simple splitting of lineages?Can we apply the GLSC to extinct species, where it’s very hard to know anything about the populations from which fossils come, and members of the same lineage could conceivably be separated by millions of years of evolution?

Have we really embraced the GLSC

For me,one of the major lessons of the GLSC should be to stop fighting over th One True Species Concept, and instead focus on gathering evidence that allows us to determine if populations are evolving independently. This idea is paid a lot of lip-service, but in practice species delimitation studies rely heavily on phylogenetic and population genetic methods. Is this really integrative?

With bacteria, viruses, and other microbes the definition(s) used for species or genus are also interesting. In some cases it is also impossible to define what a “population” is, and the degrees of isolation between populations form a wide spectrum. Some virologists have suggested we can define “species” of viruses simply by a genetic distance criteria. But it is clear that not all groups or types of viruses would best served by a one size fits all taxonomy/nomenclature system.

In the legal realm, where legislators and lawyers cannot be expected to understand the intricacies of population genetics and biodiversity, clear definitions are needed. An “endangered population” is less likely to get public support and legal protections than an endangered species. But likewise an endangered species of mammal is more likely to get attention than an endangered species of gymnosperm or bacteria. Making the legal definition of species conform to the currently accepted nomenclature/taxonomy definition within the group of organisms under discussion could allow different criteria for different types of organisms, without being completely arbitrary.

To quote: “Individual vertebrate populations can be listed as well as whole species, and listed vertebrates are protected on both public and private lands… [However,] individual invertebrate populations cannot receive protection. For example, the monarch butterfly occurs in two populations: one migrating to California and the other to Mexico for winter. Although the Mexico-wintering population is declining due to habitat destruction, it cannot receive protection under the ESA as long as the California-wintering population is abundant. The bald eagle, on the other hand, receives protection in all contiguous 48 States despite its numbers in Alaska.”